1. Field of the Invention
The present invention relates to a power supply device including a plurality of battery cells that are connected to each other through metal plates, and a vehicle including this power supply device. In particular, the present invention relates to a large-current power supply device that can be suitably used as a power source that stores electric energy for electric motors for driving electric vehicles such as hybrid car, fuel cell electric car, electric car and electric motorcycle, or for home use or industrial use, and a vehicle including this power supply device.
2. Description of the Related Art
In the case where a power supply device includes a number of battery cells that are connected to each other in series, the output voltage of the battery system can be high. On the other hand, in the case where a power supply device includes a number of battery cells that are connected to each other in parallel, the battery system can be charged/discharged at a large amount of current. In large current, high output power supply devices that can be used as power supply for an electric motor for driving a car, a plurality of battery cells are connected to each other in series so that the output voltage can be high. The power supply devices to be used for this type of application are charged/discharged at a large amount of current. Accordingly, the plurality of battery cells are connected to each other through metal plates having a small electric resistance.
See Japanese Laid-Open Patent Publication No. JP H05-343,105 A.
In the case where a power supply device includes a plurality of battery cells that are arranged side by side and connected to each other in series and in parallel, the electrode terminals of the of battery cell are connected to each other through bus bars that are formed of metal plates. For example, in the case where the battery cells are rectangular battery cells including rectangular exterior containers, when a battery assembly is constructed of the battery cells, which are arranged side by side, the electrode terminals of the battery cells adjacent to each other are connected to each other through elongated strip-shaped bus bars. In the case of a battery assembly shown in FIG. 29, battery cells 1 are connected to each other in three-cell-parallel and five-cell-serial connection through bus bars BB.
In order to detect abnormal conditions of the power supply device, not only an entire output voltage of the power supply device but also voltages of battery cells or blocks are detected. The battery cells can be assigned to the blocks. For example, in a power supply device that includes battery cells of rechargeable lithium-ion batteries that are arranged side by side, in order to detect voltages of the battery cell, each of the bus bars is connected to a voltage detection line.
However, the inventors found that, in the voltage detection through the bus bars, the resistances of the bus bars may produce measurement errors. Since conventional bus bars are formed of copper or aluminum having a small electric resistance, the electric resistances of the bus bars were considered very low as negligible. However, the inventors found according to their experiment that deferent positions of a voltage detection tab connected to the bus bar produced different voltage drops, which are produced by the bus bar and superimposed on a voltage of the cell.
Electric resistance is proportional to the length of the bus bar. In the case where the bus bar is long, the electric resistance component produced by the bus bar varies depending on the longitudinal connection position connected to the bus bar. In particular, the length of the bus bar will increase with the number of the battery cells that are connected in parallel to each other. In recent years, the power and capacity of power supply devices increase. Correspondingly, the number of battery cells included in such recent power supply devices increases. In this case, if even one battery cell is brought into an abnormal condition, this battery cell may affect other battery cells. For this reason, particularly in the case where a number of battery cells are included, it is important to monitor conditions of battery cells from the viewpoint of safety. Accordingly, in order to accurately grasp conditions of the battery cells, it is required to more precisely detect voltages of the battery cells.
In addition, there is a problem that corrosion may occur in the case where the metal bus bars are formed of a single material. For example, in a power supply device disclosed in JP H05-343,105 A, as shown in FIG. 30, the both ends of a metal plate 32 of the bus bar are fastened to the electrode terminals of battery cells 31 by nuts. That is, the electrode terminal is inserted into a through hole of the metal plate 32, and the nut is screwed onto the threaded part of the electrode terminal so that the metal plate 32 is fastened to the electrode terminal. In this power supply device, if the positive/negative electrode terminals of the battery cell 31 are formed of different metal materials, at one of the contact boundaries between the metal plate 32 and the positive/negative electrode terminals, the metal plate and the electrode terminal cannot be formed of the same material. For example, in the case where the positive/negative electrode terminals that are formed of copper and aluminum, respectively, as different metal materials, are included in a rechargeable lithium-ion battery, if the metal plate is a copper plate, different metal materials are in contact with each other in the contact surface between the metal plate and the electrode terminal of aluminum. In such a power supply device, which has a contact surface between the metal plate and the electrode terminal of different metal materials, electrolytic corrosion may occur in the contact surface between different metal materials. For this reason, there is a problem that the metal plate and the electrode terminal cannot be in stable contact with each other for a long time. When electric current flows between the different metal materials, this electric current will electrically decompose the materials so that the electrolytic corrosion occurs.
To address this, as shown in FIG. 31, a bus bar has been proposed which includes a clad plate of different metal materials that are joined to each other. For example, the bus bar is formed of a clad plate that is formed of copper and aluminum plate parts. The aluminum plate part is brought in contact with the positive electrode, while the copper plate part is brought in contact with the negative electrode for electric conduction (see Japanese Laid-Open Patent Publication No. JP 2011-060,623 A).
However, there is a problem that, in the case where voltages are detected through this type of bus bar, which includes a clad plate of different metal materials that are joined to each other, in particular, when the bus bar is long, the electric resistance component produced by the bus bar in a detected electric resistance varies in accordance with the position of a voltage detection terminal (voltage detection tab) in the bus bar. A clad plate has different electrical conductivities, i.e., electric resistances corresponding to the different materials of the clad plate. The error will be much more complicated. For example, the electrical conductivity of aluminum is approximately 60% of copper. For this reason, even if the aluminum and copper parts have the same shape, the plates parts have different electric resistances. Also, in the case where a copper voltage detection tab is used, when the voltage detection tab is joined by laser welding to the bus bar of clad plate that is formed of aluminum and copper, it will be necessary to connect the voltage detection tab not to the aluminum side but to the copper side in consideration of material characteristics. Accordingly, the connection position of the voltage detection tab will be located not at the center but at a deviated position deviated toward the copper side on a side surface of the clad plate. As a result, there is a problem that the impedances between the electrode terminals and the voltage detection tab will be different from each other.
The present invention is aimed at solving the problems. It is a main object of the present invention is to provide a power supply device that can suppress errors in battery cell voltage detection and precisely detect battery cell voltages, and a vehicle including this power supply device.